1. Field of the Invention
The present invention relates to an RFID (Radio Frequency Identification) tag, and more particularly, relates to an RFID tag including an electric or magnetic field coupling casing.
2. Description of the Prior Art
The technology of radio frequency identification (hereafter: RFID), also called electronic tag, is a communications technology for identifying a certain target and reading/writing related data via radio signal. One of the advantages of the technology of RFID is that targets are identifiable without mechanical or optical contacts.
The technology of RFID includes two parts: one is an RFID reader and the other is an RFID tag. The RFID reader is for transmitting electromagnetic wave signals to the RFID tag; an identification wave reflects or transmits from the tag, and the identification signal is sent back to the RFID reader.
There are three types of RFID tags depending on whether a power supply is built in: active RFID tags, semi-active RFID tags and passive RFID tags. Among them, passive RFID tags are not built in with a power supply and the circuit inside can only be driven via the received energy of electromagnetic wave outside. Once the passive RFID tag receives enough intensity of signals, it backscatters an identification signal to the RFID reader.
Since a passive RFID tag is not built in any battery power, it has advantages such as lower price and smaller size; therefore, it is the most widely used in the market in terms of electronic toll collection, warehouse management, logistics management, locating objects out of sight, etc.
Moreover, RFID tags are also categorized in four types according to their working frequency low frequency (LF) tags, high frequency (HF) tags, ultrahigh frequency (UHF) tags and microwave (MW) tags, wherein UHF refers to working frequency ranging from 860 MHz to 960 MHz. The higher the working frequency is, the higher the transmission rate of signals is. As a result, UHF tags win people's attention mostly and are mainly used in fields of logistics and objects locator.
However, passive RFID tags in the prior art are planar RFID tags on the basis of dipole antennas; gain of its receiving and transmitting signals is too low and is affected easily by the environment.
Besides, a UHF RFID tag is very sensitive to metal or liquid surroundings due to its characteristic of backscatter of electromagnetic wave. When the UHF RFID tag is applied on surfaces of metal, liquid or earth, the electromagnetic wave will be highly and seriously interfered or will be absorbed. The phenomena may cause the RFID chip lacks of power to get driven. Even there is enough power to drive the chip, the same phenomena may have not enough power backscatter to the reader.
One way to resolve the issue mentioned above is to separate the RFID tag from the surface of metal at an appropriate distance, or add an absorber layer on the back of the RFID tag to decrease the interference in the RFID tag caused by the metal objects. As a result, the RFID tag can only be read from its front side. Hence, the conventional way can not satisfy the market needs and can not really solve the problem.
Another way is applying a ceramic substrate with a micro-stripe antenna. The size of an RFID tag is shrunk because of high dielectric constant of ceramics. However, since the quality factor as its central frequency divided to bandwidth of this conventional RFID tag is very high, the RFID tag usually can only be applied in a range of narrow-bandwidth. Moreover, this antenna requires matching circuit between micro-strip line and chip in order to obtain the optimum effects. The matching circuit is easily affected by the ambient objects, such as metal so that the conventional RFID tag does not satisfy the market needs.
In addition, the environments for storing and managing materials or products are usually in great demand; i.e., steel bars are stacked disorderly; gas bottles of different sizes are stored together and etc. Obviously, the electromagnetic wave signals for reading the RFID tags are mostly interfered, reflected and absorbed, and the RFID tags are also likely to be damaged, which make it difficult to read the RFID tags.